The objective is to investigate the motor innervation of normal and reinnervated muscle spindles in the cat tenuissimus muscle. Light microscopic histochemical techniques (staining for cholinesterases) will be used to obtain quantitatively precise figures for the detailed pattern of the distribution of diverse motor nerve terminals to the three types of intrafusal muscle fibers in a spindle. Employing a special technique, the ultrastructural appearance of motor nerve terminals at defined sites on nuclear bag 1, bag 2 and nuclear chain fibers of the same tenuissimus spindle will be described. The sites of motor nerve terminals on intrafusal fibers will be spacially correlated with the regional fiber histochemical (ATPase) and ultrastructural profiles. The distribution of motor terminals along intrafusal fibers of the reinnervated tenuissimus spindle will be determined by light microscopy in staining for cholinesterases. In this way, the extent of selectivity with which regrowing motor axons reconnect with their original intrafusal fiber types will be examined. The study will help to understand how the function of the mammalian muscle spindle is modulated by motor nerves, and what role, if any, motorneurons play in the regulation of the structural properties of intrafusal muscle fibers.